The present invention relates to a composite substrate for use as a package or various other substrates for electronic devices such as ICs and semiconductor devices. The present invention also relates to a process for producing such a composite substrate.
Recent advances in the technology of electronic devices such as ICs and semiconductor devices are remarkable and active efforts are being made to reduce the size of such electronic devices while increasing their packing density. One of the problems that accompany these efforts is how to effectively dissipate the heat generated in devices. Conventionally, alumina (Al.sub.2 O.sub.3) packages or substrates have been used with micro-electronic devices but in order to achieve more efficient heat dissipation, various new types of substrates have been proposed and reviewed. Among them are: (1) a ceramic substrate made of a material having good heat conductivity such as BeO, AlN or SiC; (2) and enamelled substrate consisting of an iron sheet with an enamel coating; (3) a metal substrate to which an insulator is bonded with an adhesive; (4) a metal substrate on which a ceramic powder is flame-sprayed; (5) a metal substrate on which a thin ceramic film is formed by a suitable method such as physical vapor deposition (PVD) or chemical vapor deposition (CVD); and (6) a metal substrate on the surface of which an insulating organic polymer layer is formed. However, these substrates have their own problems described below and are not completely satisfactory for use in practical applications.
(1) A ceramic substrate made of a good heat conductor such as BeO, AlN or SiC conducts 5-20 times as much heat as an alumina substrate but its manufacture requires a complicated process comprising purification of the starting powder, controlling the particle size of the powder, shaping the powder into a compact and sintering the compact. In addition, the need for performing sintering at elevated temperatures (1500.degree.-2000.degree. C.) involves such disadvantages as difficulty in preparing a substrate of large surface area, development of thermal distortion and high cost. Of the three material mentioned above, BeO has the highest heat conductivity but because of its toxic nature and high cost it can be used in only a very limited area.
(2) The manufacture of an enamelled substrate involves fusing enamel frit in the high temperature range of 650.degree.-800.degree. C., so that the resulting enamel layer as an insulator has a minimum thickness of 0.5 mm and reduces the heat conductivity of the substrate. A thin (.ltoreq.0.1 mm) enamel coat contains so many pinholes in its surface that its insulation withstand voltage will fall to a commercially unacceptable level.
(3) A metal substrate to which an insulator such as alumina is bonded with an adhesive has not yet been commercialized for several reason such as an increased resistance to heat conduction in the adhesive layer and unevenness in the adhesive strength of various parts of the substrate.
(4) A metal substrate on which a ceramic powder is thermal-sprayed contains so many pinholes in the sprayed insulation layer that it does not have sufficient insulation withstand voltage or the desired surface flatness of insulation layer.
(5) A metal substrate on which a thin ceramic film is formed by a suitable method such as PVD or CVD requires heating at 500.degree. C. or higher in order to form the thin ceramic film and this causes such disadvantages as a decreased freedom in the choice of a suitable metal substrate and a reduction in the strength of the substrate. In addition, the adhesion between the thin ceramic film and the metal substrate is not strong and a great amount of unevenness is introduced in the film quality.
(6) A substrate having a thin organic polymer layer is also disadvantageous in that polymers have poor heat resistance and are not highly heat-conductive to serve as an efficient heat dissipater.
Metal layers that establish connection to the device or provide an inter connecting circuit are usually formed on the insulator (or insulating layer) of each of the substrates described above. Conventionally, these metal layers are formed by techniques such as bonding with an adhesive, a thick-film process, PVD and CVD. A problem common to these techniques is that the metal layer does not adhere strongly to the insulation layer and separation between the two layers may readily occur if thermal stress is produced by for example, the soldering of lead wires. If one wants to provide stronger adhesion between the metal layer and the insulating layer, precise process control must be performed but then this results in high manufacturing cost.